S = map(int, input().split())
S = list(S)
K = int(S[0])
M = int(S[1])
F = int(S[2])

A = input()
A_split = A.split()

sum = K * M
toshi = 0
for i in range(F):
    toshi = toshi + int(A_split[i])

if sum >= toshi:
    ans = sum - toshi
else:
    ans = - 1

print(ans)